Underground liquid storage tanks are often used to store corrosive and/or environmentally hazardous liquids such as gasoline. These tanks have typically been fabricated from steel or fiberglass reinforced plastic resin ("FRP"). In recent years, double-walled tanks have been developed which provide an annular space between an inside wall and an outside wall for secondary containment of any leakage, either from outside the tank into the tank due to a break in the outer wall, or from inside the tank out of the tank due to a break in the inner wall. It is well known that the annular space can be filled with a leak detecting fluid which can then be monitored to detect the occurrence of a leak in the tank's inner or outer wall ("hydrostatic" or "wet" systems). Alternatively, the annular space between the walls can be left empty and provided with a liquid detector suitable for detecting liquid leakage ("dry systems").
Two important considerations in developing improved double-walled storage tank manufacturing methods and designs are (1) reducing the cost of manufacturing the tanks, and (2) providing a design which allows for rapid transmission of leaking liquid into and through the annular space.
One prior proven double-walled tank design, disclosed in U.S. Pat. No. 4,923,081, issued to Weaver et al., comprised generally parallel inner and outer tank walls separated by the inner wall's circumferential support ribs. While this design was adequate, fabrication, particularly fabrication of the outer wall, was relatively time-consuming and costly. In addition, this design required a relatively thick outer wall since the outer wall was not buttressed by support ribs. Also, significantly more detecting fluid was required to fill the larger annular space in wet systems.
One obvious design improvement was an outer wall which followed the profile of the support ribs and inner wall, thereby reducing the volume of the annular space (and thereby the cost of detecting liquid required to fill the annular space). Double-walled tanks incorporating this improved feature were contemplated, and even patented. However, no automated, cost-effective fabrication method was developed to create the outer wall which conforms generally to the profile of the inner wall and support rib until Owens-Corning Fiberglass Corporation developed a method and apparatus for compressing the laid-up outer wall layers into the space between the support ribs as the outer wall layers were applied. This improved fabrication method (disclosed hereinafter in greater detail) resulted in the commercialization of the double-walled tank design disclosed in U.S. Pat. No. 5,155,936, issued to Bartlow.
As illustrated by the Bartlow patent, it is known to interpose a plastic mesh spacer material between the inner and outer walls of a tank between the support ribs to define the annular space. Bartlow also discloses the connection of the annular space with the hollow space inside the support ribs to allow for communication of leak detecting fluid and/or leaking liquid from the annular space to the interior of the ribs.
Thus, previous double-walled tank designs included spacer material to define an annular space adequate for rapid transmission of leaking liquid into and through the annular space, and therefore relatively rapid detection of leaks. However, the annular space, particularly where such space communicated with the hollow support ribs, was still sufficiently large that a significant volume of leak detecting fluid was required in order to fill the space. It was thus desirable to develop a double-walled tank design which further reduced the size of the annular space.
One obvious way of further reducing the annular space was to eliminate the spacing material between the inner and outer walls. Each of U.S. Pat. Nos. 4,653,312; issued to Bruce R. Sharp, and Re. 34,203; issued to David T. Palazzo, disclose double-walled storage tanks wherein the outer wall is fabricated directly over the inner tank after the inner tank has been coated only with a release agent. These proposed designs rely upon capillary action for the migration of liquid in the extremely small annular space between the inner and outer walls of the tank. And, while such migration does occur, it is extremely slow, resulting in the problem of long-delayed detection of fluid which has leaked into the annular space. Another problem with the proposed designs is the inordinate time required for filling the annular space with leak detecting liquid for those double-walled tanks which utilize hydrostatic systems.
Thus, while the prior art evidences the obvious evolution of double-walled tank design towards continuing reduction of the annular space between the inner and outer walls, no one has yet disclosed a commercially practical design and method for making a fiberglass reinforced resin double-walled storage tank that does not require spacing material between the walls for commercially feasible fabrication, installation, and leak detection.